You need to compile the less files into css files. 
```
$ cd ipython/IPython/html
$ fab css
[localhost] local: components/less.js/bin/lessc -x  style/style.less style/style.min.css
[localhost] local: components/less.js/bin/lessc -x  style/ipython.less style/ipython.min.css

Done
```

You might need extra dependencies like `fabric`,`node` and `lessc` to do so still.